There are some technical disadvantages related to different means for transferring electrical power, such as power transmission lines and power transmission cables, especially lossy and capacitive power transmission lines or cables.
Undersized power transmission lines for electric power are often referred to as “weak” lines. Such lines have a too small conductor cross section in relation to the load requirement, and consequently a relatively high resistance and hence high series impedance. Long transmission lines may also be capacitive.
Power transmission cables for transferring AC current may only be used up to a certain distance at high voltage levels, since their capacitive properties will prevent power transfer when the cables reaches a certain length. Long cables are also lossy, i.e. they have high series impedance.
Normally, a capacitive line or cable must be compensated to avoid/reduce the Ferranti effect. Moreover, a lossy line or cable may provide an excessive voltage drop resulting in inadequate voltage levels for the load connected to the line or cable.
Subsea power transmission cables are typically capacitive and lossy. Due to these properties, the length of such cables is limited. A step-up transformer may be provided in the end of the cable for transforming the voltage up to an acceptable level for the load during nominal load. However, if the load fails or is significantly reduced, the Ferranti effect will cause the cable end voltage to increase, possibly over allowed limits for the cable, penetrators or transformer. Hence, the load voltage will reach an unacceptable high level which may damage the load.
If the load is a subsea pump, the load may vary between zero and to a nominal load. Moreover, if a variable frequency converter is used to control the speed of the pump, the reactive power drawn from the cable will vary according to frequency.
In FIG. 1, corresponding to FIG. 43 of WO 03/044613, it is shown a magnetic device comprising a magnetic core, a first winding and a second winding wound around the core, and a control winding wound around the core for controlling the permeability of the core. The device may be used as a transformer with controllable magnetizing inductance.
The object of the present invention is to provide a power transmission system where the length of the capacitive and lossy power transmission cable can be increased.
Moreover, it is an object of the invention to provide a power transmission system which may handle load variations between zero and nominal load.
Moreover, it is an object of the invention to provide a power transmission system which may handle frequency variations of the input power between zero and a nominal frequency. In such situations, it should be possible to connect the power transmission system to a standard (i.e. off the shelf) frequency converter, that is, the power transmission system should not be dependent on the frequency converter or vice versa.
It is also an object to avoid power electronic components, i.e. semiconductor devices, in the main current line of the power transmission system.
Moreover, it is an object to provide a power transmission system with a control system which is independent of a separate power source.